1. Field of the Invention
The present invention relates to image forming apparatuses such as electrophotographic copying machines. Specifically, the present invention relates to an image forming apparatus including an array of photosensitive members that independently form a cyan image, a magenta image, a yellow image, and a black image, and thus form a color image as a result of a combination of these primary images.
2. Description of the Related Art
A known laser image forming apparatus for forming a color image has four arrays of electrophotographic photosensitive members and corresponding laser light sources as image exposure means. Oscillation of four laser light sources is controlled based on cyan, magenta, yellow and black image signals from an image to form four electrostatic latent images on the corresponding electrophotographic photosensitive members. These electrostatic latent images are developed and recombined to form the color image.
Laser beams from four laser light sources must be scanned so that four scanning beams are synchronized in the main and sub directions and thus cyan, magenta, yellow and black images are precisely recombined to form a color image free from color offset. It is, however, difficult to synchronize scanning of the four laser beams in the main and sub directions.
Another type of known color image forming apparatus has four electrophotographic photosensitive members and four LED light sources for forming cyan, magenta, yellow and black electrostatic latent images, in place of the laser light sources. In this apparatus, four scanning beams are relatively readily synchronized. On the other hand, a plurality of expensive LED chips are rearranged into four LED arrays in the apparatus (each LED array is referred to as a combined LED device), resulting in increased production costs of the apparatus. Since individual LED chips have different light emission characteristics, exposition by a combined LED device differs in the main scanning direction. Thus, image reproducibility is unsatisfactory in the main scanning direction.
In the color electrophotographic-copying machine, a combined LED device must be arranged for each of four electrophotographic photosensitive members. Since four combined LED devices also have different light emission characteristics as described above, it is even more difficult to precisely reproduce the original image.